Characteristics of strong mine pressure manifestation in composite hard roofs and determination of main controlling layers

The interactions within the composite hard roof structure above coal seams are highly complex, making it challenging to identify the main controlling layers for pressure relief via hydraulic fracturing. Using the 31104-1 working face of the Nalinhe No. 2 Mine at Wushenqi Mengda Mining Co., Ltd. as the engineering background, this study employed physical similarity simulation, theoretical analysis, and engineering tests to investigate the fracture and energy evolution patterns of composite hard roofs. The study revealed the mechanism behind strong mine pressure in composite hard roofs and identified the main controlling layers for hydraulic fracturing. The results showed that during the initial mining phase of the composite hard roof working face, the collapse was concentrated in the lower hard roof, with periodic fractures creating periodic pressure on the working face. The upper hard roof, supported by collapsed gangue below, failed to collapse fully. When the working face advanced to a square position, the upper hard roof and the intervening strata collectively fractured, causing strong mine pressure. After directional pre-splitting of the upper hard roof, the overburden exhibited a typical "three-zone" structure, allowing the upper roof to fracture and sink promptly. At the square position of the working face, the acoustic emission ringing count of the upper roof decreased by 38.36% compared to pre-splitting of the lower hard roof. The energy in the concentrated microseismic event area was reduced to 1000-2000 J, while the total microseismic energy, single-pick energy, and number of events decreased by 62.17%, 71.92%, and 56.32%, respectively, compared to the pre-splitting of the lower hard roof. These findings confirmed that the upper hard roof was the main controlling layer for pressure relief, and pre-splitting the upper hard roof effectively suppressed strong mine pressure on the working face.